The present invention relates generally to optical systems more particularly to an optical system providing concurrent calibration signal and test signal detection in an optical spectrum analyzer that is useful in analyzing optical telecommunications transmission lines.
The telecommunications industry is increasingly deploying dense-wavelength-multiplexed (DWDM) optical system in their optical networks. A typical DWDM optical system launches multiple optical signals at various wavelengths into a single mode optical fiber. The optical signals include a 1480 nm optical signal from a pump laser. The pump laser signal is used for fiber amplifiers in the system. The pump laser signal may also have a wavelength of 980 nm. A 1625 nm service channel optical signal is provided for communications between central offices and the like. Multiple, closely spaced optical signal channels, in the range of 1525 nm to 1585 nm, are used for telecommunication traffic through the fiber. The standard separations between adjacent optical signal channels for current DWDM transmission systems are 200 GHz., 100 GHz., and 50 GHz. which approximately equals 1.6 nm, 0.8 nm, and 0.4 nm separation between channels at 1550 nm. Future DWDM telecommunications systems are designed with 25 GHz. separation which approximately equals 0.2 nm separation between optical signal channels. To qualify and characterize these optical signal channels requires the use of an optical spectrum analyzer.
Optical spectrum analyzers (OSA) are instruments that measure the optical power as a function of wavelength or frequency. Advantages of optical spectrum analyzers are their dynamic range and performing measurements involving many discrete spectral lines. A significant drawback to existing optical spectrum analyzers is the relative unreliability of their wavelength measurements, with errors in the range of 40 to 50 picometers. Because of this drawback, wavelength meters have been developed to make precise wavelength measurements and to calibrate optical spectrum analyzers. Wavelength meters are based on the Michelson interferometer. Thousands of digitized interference fringes are converted from the spectral domain to the frequency domain. The frequency and modulation of the fringes are converted through the Fourier transform into information about the wavelength and power. While wavelength meters have much better wavelength calibration accuracy, they typically have much worse dynamic range than a grating-based OSA.
Generally, the measured optical signal and the calibration optical signal both follow the same optical path through the optical spectrum analyzer and occupy the same general region of the optical spectrum. A typical calibration of an OSA uses the following procedure. First, an optical signal with a known spectra is applied to the OSA from a calibration source. The calibration source may be external to the OSA or it may be an internal source that is injected into the OSA""s optical path through an internal optical switch. The optical spectra is scanned with the OSA and the wavelengths at which the spikes occur in the known spectra are measured and recorded. The wavelength errors at the measured spikes are determined and the wavelength-measurement errors are estimated as a function of the wavelength by interpolating between and beyond the known spectral lines. The wavelength-measurement errors are subtracted from the corresponding measured wavelength spikes to calibrate the OSA.
Because the calibration optical signal and the measured optical signal both follow the same optical path through the OSA, the OSA cannot measure the calibration spectra at the same time it is being used to measure an unknown optical signal. Therefore, the calibration procedure is a serial process of calibrating the OSA and them measuring the test signal. The OSA is presumed to remain calibrated for a certain amount of time, whereupon it must be calibrated again before proceeding with further measurements.
One drawback to the current calibration procedure is the uncertainty in knowing when the OSA is out of calibration. This means that recalibration typically occurs either before it is necessary, or after it is necessary. In the first case, an operator wastes time with an unnecessary calibration and in the second case the measurement results of the OSA have excessive error because the calibration was not performed.
What is needed is a optical system providing concurrent optical detection of an optical test signal and an optical calibration signal in an optical spectrum analyzer. The optical system should provide very accurate wavelength calibration in the optical spectrum analyzer. Additionally, the optical spectrum analyzer should be capable of concurrently detecting both the calibration signal and the optical signal under test using two optical paths with the same wavelength calibration characteristics. Further, the optical spectrum analyzer should provide optical isolation between the calibration signal and the optical test signal.
Accordingly, the present invention is to an optical system having a defined first order spectral range that concurrently detects an optical calibration signal and an optical signal under test. The optical system has collimating optics, such as a parabolic or spherical mirror or the like, having an optical axis and a focal plane that receives the optical calibration signal and the optical signal under test. A fiber array is disposed in the focal plane of the collimating optics and has a center axis that is colinear with the optical axis of the collimating optics. First and second pairs of optical fibers are disposed in the focal plane of the collimating optics with each pair of fibers having an input optical fiber and an output optical fiber. The input and output optical fibers of each pair are symmetrically positioned on either side of the center axis with the input fiber of the first pair of optical fibers coupled to receive the optical signal under test. An optical source is coupled to the input fiber of the second pair of optical fibers and generates an optical calibration signal having second order or greater spectral lines that fall within the first order spectral range of the optical system. An optical tuning element receives the optical calibration signal and the optical signal under test from the collimating optics and tunes the optical system over the first order spectral range to separate spectral components of the optical calibration signal and the optical signal under test. A first optical detector is coupled to the output optical fiber of the first pair of optical fibers and is responsive to the spectral components of the optical signal under test and less responsive to the second order or greater spectral lines of the optical calibration signal. A second optical detector is coupled to the output optical fiber of the second pair of optical fibers and is responsive to the second order or greater spectral lines of the optical calibration signal and less responsive to the spectral components of the optical signal under test.
In the preferred embodiment of the invention, the fiber array is a V-groove block having approximately V-shaped channels formed therein that are parallel to and equidistant on either side of the central axis of the V-block. The optical source is a optical signal generating device producing a spectral output in response to shifts in emission or absorption energy levels in atomic or molecular species. In the preferred embodiment, the optical source is a mercury-argon discharge lamp. The optical tuning element is preferably a diffraction grating. The first optical detector is a InGaAs PIN or avalanche photodiode that is responsive to the first order spectral components of the optical test signal. The second optical detector is a silicon photodiode that is responsive to the second order or greater spectral lines of the optical calibration signal.
The various embodiments of the optical system may be incorporated into an optical spectrum analyzer concurrently detecting an optical signal under test and an optical calibration signal. Such a combination provides continuous calibration of the optical spectrum analyzer. The optical spectrum analyzer includes the optical system where the optical test signal detector and the optical calibration signal detector are included in respective optical receivers. The optical receivers convert the respective optical signal into electrical signal that are converted to digital values. The digital values representing the optical test signal and the optical calibration signal are processed by a controller, such as a digital signal processor, and an calibration error value is calculated based on the second order or greater spectral lines of the optical calibration signal. The calibration error value is applied to the optical signal under test to accurately calibrate the test signal. The optical test signal is further processed and displayed on a display device.
The method of concurrently detecting an optical calibration signal and a optical signal under test in an optical spectrum analyzer having an optical system with a defined first order spectral range and comprising collimating optics, an optical signal calibration source generating an optical calibration signal, an optical signal under test input, an optical calibration signal input, an optical tuning element and first and second optical detectors, includes the step of concurrently launching the optical calibration signal and an optical signal under test into the optical system wherein the optical calibration signal has second order or greater spectral lines that fall within the range of the optical system. The method also includes the step of concurrently detecting the optical calibration signal and the optical signal under test using the first optical detector that is responsive to the optical signal under test and less responsive to the second order or greater spectral lines of the optical calibration signal and the second optical detector that is responsive to the second order or greater spectral lines of the optical calibration signal and less responsive to the optical signal under test. A further step in concurrently detecting an optical test signal and an optical calibration signal includes tuning the optical system over the first order spectral range to separate spectral components of the optical calibration signal and the optical signal under test. The method includes the additional step of concurrently converting the optical calibration signal and the optical signal under test to electrical signals.
The objects, advantages and novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawings.